Of particular interest for this disclosure is the communication between devices that are based upon the Institute of Electrical and Electronics Engineers (IEEE) 802.11 technology, commonly known as Wi-Fi. IEEE Standard 802.11-2012 may be used as the basis for the specifications used in this disclosure.
The following is a review of the protocol layers that are present in a typical Wi-Fi device so that descriptions of the proposed disclosure can be readily understood. FIG. 1 is a block schematic diagram that represents two communicating Wi-Fi devices 110 and 120. Wireless device 110 includes a logical link control (LLC) layer 111, a media access control (MAC) layer 112 and a physical layer (PHY) 113. Similarly, wireless device 120 includes a logical link control (LLC) layer 121, a media access control (MAC) layer 122 and a physical layer (PHY) 123. The LLC layers 111 and 121 allow further protocol multiplexing over an Ethernet connection. Consider the transmission of data from device 110 to device 120. In wireless device 110, the MAC layer 112 accepts data, in the form of MAC service data units (MSDU) 114 from the LLC layer 111. The MAC layer 112 is the function that arbitrates the use of the network capacity and determines which stations are allowed to use the medium for transmission. The MAC layer 112 sends the MSDU data 114 in the form of a physical layer convergence procedure (PLCP) service data unit (PSDU) 115 to the physical layer (PHY) 113. The PHY 113 is responsible for transmitting the PSDU data 115 to the other device 120 in the form of a PLCP protocol data unit (PPDU) 131.
The PHY will typically comprise radio frequency (RF) transmitters and RF receivers. The actual numbers of transmitters and receivers are not limited to what is shown in FIG. 1 or described in this disclosure and may depend upon the different PHYs that are described in the IEEE 802.11 specification. The PPDU 131 includes the PLCP headers, MAC headers, the MAC data field and the MAC and PLCP trailers. The MAC protocol data unit (MPDU) 130 is the frame PPDU 131 without the PLCP headers. Therefore, the MPDU 130 can be considered as the data unit that is exchanged between the MAC layers 112 and 122 where the PPDU 131 is what is exchanged over the wireless medium. The PPDU 131 is received at the PHY 123 in device 120 which then sends the resulting PSDU 125 up to the MAC layer 122. The MAC layer 122 in turn sends the resulting MSDU 124 up to the LLC layer 121.
FIG. 2 is a diagrammatic representation depicting an embodiment of a monitoring wireless station (STA) 210 that is located in an airborne platform 200. Connected to the monitoring STA is an antenna 220. The antenna 220 may be a directional antenna. The area 230 represents the area on the ground that the antenna 220 is effectively covering in that signals within the area 230 have the potential of being received by the monitoring STA 210 through its accompanying antenna 220. Area 230 may include a number of Wi-Fi networks 240. The networks 240 may be infrastructure Wi-Fi networks each having one or more APs and several STAs. The networks will in practice be communicating on a selection of channel frequencies, and several networks may be using the same channel frequency. It should be noted that due to the propagation losses between the networks 240 on the ground, networks using the same frequency do not necessarily overlap and hence will be transmitting packets simultaneously. At the airborne monitoring STA 210, however, due to the predominantly line of sight propagation, that same simultaneous network traffic on the same frequency channel is more likely to be received, i.e., reception of non-overlapping network traffic will be effectively overlapping at the monitoring STA 210.
The following is a discussion of the standard reception method for monitoring wireless packets in a typical Wi-Fi device. FIG. 3 is a timing diagram showing a number of transmitted packets from a number of networks as received by a legacy monitoring receiver. In this example, at time T1 360 packet 341 that was transmitted by network D 340 is received by the monitoring STA. At time T3 362 packet 341 is complete. In order to receive a new packet, the start of the packet must be detected, hence the next packet that may be received is packet 332 from network C 330 at time T5 364. At time T6 365 packet 311 from network A 310 is detected by the monitoring STA. In this example, packet 311 from network A 310 is received at a higher signal strength than packet 332 and in this case, the monitoring receiver may abandon the reception of packet 332 and start receiving packet 311. This is known as step-up where a receiver may abandon the reception of a packet if the start of a new packet is detected at a significantly higher signal strength.
At time T7 366, packet 311 is complete and the next packet that the monitoring receiver may receive is then packet 342 from network D 340 at time T9 368. It should be noted that for the monitoring receiver to detect and start to receive a new packet, it is generally necessary for there to be a clear period, the exception being if a new packet is detected at a significantly higher signal strength such that step-up can take place. In the example as depicted in FIG. 3, after the monitoring STA has completed receiving packet 341 it must wait until packet 321 from network B 320 and packet 351 from network E 350 have completed. There is then a free period between times T4 363 and T5 364 which is when packet 332 starts. Similarly after the reception of packet 311, the monitoring STA must wait until packet 352 from network E 350 has completed and there is a free period between T8 367 and T9 368 which is the time that reception of packet 342 commences. The example as depicted in FIG. 3 demonstrates that in the case of a single airborne monitoring receiver, when a number of signals on the same frequency channel are received from a number of ground based networks, the receiver can only detect and receive a relatively small proportion of the packets.
FIG. 4 shows two examples of the general format of IEEE 802.11 wireless packets. In FIG. 4, packet 400 is a sample packet used by a Wi-Fi transmitter in the 2.4 GHz band complying with Clauses 16 Direct Sequence Spread Spectrum (DSSS) device and Clause 17 High Rate DSSS (HR/DSSS) device in the IEEE 802.11 standard. Such devices are commonly known as 11b devices. The packet 400 starts with a preamble 401 which is followed by a header 402. The MAC header 403, the frame body 404 and the final frame check sum (FCS) are transmitted at the data rate which, in the case of an 11b device may be, for example, 1 megabit per second (Mbps), 2 Mbps, 5.5 Mbps or 11 Mbps. The preamble 401 is always transmitted at 1 Mbps. In the case that the data rate is 1 Mbps, the header 402 is also transmitted at 1 Mbps. In the cases that the data rate is 2 Mbps, 5.5 Mbps, or 11 Mbps, the header 402 may be transmitted at 1 Mbps or 2 Mbps.
In the 2.4 GHz band, it is also possible to use the orthogonal frequency division modulation (OFDM) of Clause 19 Extended Rate PHY (ERP), commonly referred to as 11g. In FIG. 4, packet 450 is in a format used by a Wi-Fi transmitter in the 2.4 GHz band complying with Clause 19 in the IEEE 802.11 standard using OFDM. The preamble 451 and the signal 452 are transmitted at the lowest OFDM rate, i.e., 6 Mbps. The service field 453, MAC header 454, frame body 455 and FCS 456 are transmitted at the required data rate.
The start of a Wi-Fi packet is at the lowest PHY rate, 1 Mbps in the case of an 11b device. The signal to noise and interference ratio (SNIR) requirement in order to detect the 1 Mbps preamble is very low, and hence the detection of the preamble and header by a receiver is very sensitive. Similarly, the detection and demodulation of a complete packet sent at 1 Mbps can be achieved in low SNR conditions. In theory, the SNIR requirement for 1 Mbps is in the order of 0 dB which means that even if the noise and interference power is at a similar power to the packet signal level, the receiver may still correctly detect the packet. Thus, it is possible to detect the start of a packet even if another packet is present as long as the SNIR is above 0 dB.
Wireless packets that are of interest to a monitoring STA include management packets as well as control packets and data packets. Of particular interest are beacons that each wireless device, i.e., AP transmits on a regular basis. Beacons in the 2.4 GHz band are generally transmitted at 1 Mbps PHY rate. Other management frames, such as probe requests and responses, authentication packets and association packets are often also sent at 1 Mbps PHY rate. It is possible to detect the start of many management packets even if another packet is present as long as the SNIR for the management packet is above 0 dB. When wireless packets are monitored, referring to FIG. 4, the preamble 401 and header 402, or the preamble 451 and signal 452 will be transmitted at a low PHY rate as previous described above with respect to FIG. 4. The MAC header field 403 or 454 is of particular interest because the address fields are contained within. The MAC header field, however, is transmitted at the desired PHY rate. For example, for an 11 Mbps data packet, the preamble 401 is sent at 1 Mbps, the header 402 is sent at 2 Mbps and the MAC header 403, frame body 404 and FCS 405 all sent at 11 Mbps. The preamble 401 and header require about 0 dB SNIR for detection, while the rest of the packet transmitted at 11 Mbps requires about 5 dB SNIR.
Consider again FIG. 3. The progressive reception of packets 341, 332, 311 and 342 is due to the monitoring STA obeying a rule that it can only detect the start of a new packet if the medium is free for a finite amount of time before the packet is received. The practical situation, however, is that if the SNIR is sufficient, then a packet may be detected and received even if another packet is active and present at the same time. Hence, at time T7 366, the monitoring STA may, at the conclusion of the receipt of packet 311, then detect the start of packet 352, even though packet 332 is also being received. In this case, at time T9 368 the monitoring STA will then detect packet 342. Hence, as shown in this example, dependent upon the relative signal strengths of packets present at the receiver, and on the timing of those said packets, the number of packets detected and received will vary.
A monitoring STA, when used to monitor transmissions, is placed into what is termed promiscuous mode. In promiscuous mode a STA acts solely as a receiver and also attempts to receive the complete packet. In a standard communications mode, a STA will detect if the packet was addressed to itself and only if confirmed will the complete packet be demodulated. In the monitoring promiscuous mode, the STA will attempt to demodulate all detected packets.
When a monitoring wireless device, or monitoring STA is located in an aircraft and is used to either monitor transmissions between devices on the ground, or to communicate with one or more ground based devices, a problem exists in picking out any specific communication or packet of interest due to the number of packets that are present at any one time. Because the STA is airborne, with one or more antennas that are directed towards the ground, it is a common condition that the area of coverage on the ground is relatively large and as such the traffic in many networks can be detected simultaneously. In this situation, a single receiver cannot be assured that it will receive the specific communication it is looking for.